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Polarity Analyzer From Monster Cable Ralph Glasgal Yes, THE Monster Cable. The Monster Car Audio division of Monster Cable has come up with a tester for absolute polarity that they market primarily to the auto sound market. Their sales brochure states "Ensuring wire polarity can be a difficult task that involves removal of car door panels or removal of the dashboard. Visibly checking speaker connections for polarity accomplishes little except verifying the factory labels on the voice coil terminals." I would have thought that at this moment in audio history, the surround sound market for a tester like this would be even more substantial than the auto sound market, since, to paraphrase, ensuring absolute polarity of a multi-speaker, multi-driver, multi-channel surround sound system can be a difficult task that involves the removal of speaker grills and speaker panels to obtain access to crossover network and speaker terminals. Unless you connect a battery or a test signal, at this juncture, just visibly checking speaker and crossover connections for polarity accomplishes little except verifying the factory labels on the speaker or other inside terminals. I have not used this tester in any of my cars, but as an audiophile, I can't imagine stationary sonic life without it anymore. Psycho-Polarity Background Have you ever wondered whether you yourself can hear absolute polarity (see sidebar) when listening either to just one speaker or two? Are you really sure that your woofer, subwoofer, midrange driver and even tweeter all jump in the same direction in response to a steeply rising musical waveform? How about your amplifiers, or preamps-do you know if they invert inadvertently? How about CD decoders or microphones? How about your surround sound system-do all six speakers (and their amplifiers) move in and out in unison, at least when they are supposed to? In the non-electrical, natural world, there is no mechanism that can cause a sound's polarity to invert. The leading edge compression of a trumpet's first utterance is still a compression when it reaches your ears, and so are all the early reflections from walls and floors that follow. The initial attack of the kettledrum can only be a rarefaction unless the percussionist is inside the kettle. If you yodel in the Alps, what goes out and then comes back may be delayed, but it is still right-side up. However, in the modern world where analog electrical waveforms represent sound, it is all too easy to turn things upside down either deliberately or in ignorance and for audiophiles this ignorance is not blissful. Two differing considerations involved with sonic polarity must be addressed. The first is the determination of absolute polarity. That is, do we know, from the kettle drum to your ears, that the leading edge of each beat is still a rarefaction. There are those who maintain that this is not important and that the kettle drum will sound the same even if the diaphragm first moves out instead of in when a recording of the drum is played. The second is determining that, where more than one channel or sound transducer is involved, that all sound-producing elements in a system are outputting sound of the same polarity even if that polarity may be wrong. While absolute polarity is probably inaudible to most individuals, relative polarity inversion among sound-producing elements clearly is audible. Fortunately the test system under review here addresses both issues, at least as far as the reproduction chain is concerned. But remember there is no guarantee that the recording engineer or CD producer maintained absolute or even relative polarity of all microphones, equalizers, mixers, etc., before encoding a particular disc. Determining Your Own Ability to Hear Absolute Polarity The Monster Cable Polarity Analyzer consists of a compact disc containing two types of test signals and some verbal tutorial discussions on using the tester, particularly in cars. The other half of the tester is the hand-held analyzer, which is about the same size as a remote control and is battery powered. It contains a microphone, pulse detection circuitry, one red LED, and one green LED. Recorded on this CD are two 20-minute bands of pulse trains, one high and one low. The first high band is for determining the polarity of the higher frequency drivers of multi-way speaker systems or full-range electrostatics and ribbons. The test signal consists of three narrow but rounded positive pulses followed by a single negative pulse. When one places the hand-held acoustic sensor in front of the driver being tested, an LED blinks each time a pulse is reproduced. The green LED lights for positive acoustic pulses and the red lights for the negative pulse. If the polarization of your sound system is correct, every driver in every speaker should produce a three-green, one-red pattern. Note that this tester cannot be used to directly check the polarity of only an amplifier, since it has no input jack. However, once you have a small speaker whose polarity you know is correct, you can use it as part of the test kit for amplifiers, and once you have an amplifier you are sure of, you can use it and your speaker to test just about anything in the chain except cartridges.You can even check microphone chains by playing the CD through one correctly polarized speaker placed directly in front of the microphone and the listening with the analyser to a second previously checked monitor. For those who want to see if they can hear absolute polarity under these circumstances, try to identify which of the four pulses is the one that is reversed. Try this experiment using a speaker with just a single driver and then with speakers with multiple drivers and finally with stereo speakers. This is a true double-blind test because the pulses are evenly spaced, and there is no way you can keep track of which pulse is which without using the analyzer. I, myself, can hear no difference but that doesn't mean you won't. Please try this and E-Mail your results to ralphglasgal@webtv.net, or post them on rec.audio.high-end Testing Subwoofers The high-band pulses are only about a half millisecond wide and repeat at about a half-second rate. Such pulses are too short to influence a woofer and so a second band of pulses about twice as wide is provided to test the polarity of middle-range woofers. Polarizing woofers and even subwoofers in my own system turned out to be a cinch. The pulse provided is actually too narrow to be a good test signal for a deep subwoofer, so it is a good idea to momentarily change the crossover frequency to something over 80 Hz. to be sure there is enough of a pulse edge for the detector to reliably sense. This raises a weakness of this method. When nice pulses like these go through a crossover network and then into a complex load like a tweeter, the pulse may be differentiated, producing two pulses of opposite polarity, ringing, or other radical alteration. Thus with some electrostatic speakers I tested (admittedly driven by some very complex digital signal processors) I could detect no negative pulse at all. However, one could argue that a system that cannot accurately reproduce this simple series of pulses is not of audiophile caliber. Really complex four-or five-way systems should also be checked by placing the tester at the listening position so that the transient acoustical responses of the different drivers have a chance to add up correctly at the proper distance from the speaker array. One can hope that at some point the Monster will provide a CD with a real cornucopia of even more useful pulse trains. One Man's Results Since in the past I used a battery and my hand, or an oscilloscope to check all new additions to my system, I was pleased to confirm that all 20 drivers in my system were in lockstep relative to each other. Imagine my surprise then to find that my CD decoder was upside down! Fortunately it has a polarity inversion switch and so I didn't have to move to Australia. On the other hand-how can one tell if a CD is of the correct polarity? Again, throwing the absolute polarity switch is inaudible to me and may make no difference to you, but having it right makes it easier to keep track of the other relative polarity things in the system and you may have friends who can hear absolute polarity just as some musicians have absolute pitch. On the relative polarity side, having one element upside down in a multichannel system will cause quite audible, diffuse, imaging problems or loss of bass response. The bottom line is that Monster Cable has provided a really useful gadget for determining the absolute polarity of almost any speaker or amplifier-speaker combination at a reasonable price of $120 dollars from any Monster Cable dealer; that includes a rather hefty black carrying case if you decide to make house calls to doctor sick systems. It is a godsend for installing subwoofers or surround-sound systems particularly where the speakers and/or amplifiers are of different manufacture. Tutorial: Polarity vs. Phase Vs Time Delay Unfortunately, we live in an age where loose lips sink scholarship. Thus much, if not most, of the audio establishment uses the word phase when they really mean polarity. Such pedantry on my part is probably worthy of contempt but the distinction between polarity, phase shift and time delay can be significant for those struggling to get the best out of their equipment or weighing the claims made by manufacturers of time aligned speakers or phase compensated cables, for example. The difference between these wavetrain parameters is best thought of in terms of ordinary music. A musical sound, after conversion by a microphone into an electrical voltage, can be seen as a series of positive and negative spikes of varying widths and amplitudes. If we look at a particular positive spike,on its way to a loudspeaker, and delay its arrival time at our ears by moving further away from the speaker, the delayed spike will still be positive although it is attenuated and rounded, since it passes through more air. If we reverse the leads to the speaker, the positive spike becomes a negative spike and so by definition its polarity has been inverted. Note that polarity, like the concept of up and down, is a defined attribute and not a relative one. Knowing the definition (in this case rarefaction or compression) one can determine the polarity of a single speaker without comparing it to any other speaker, signal or time reference. Time delay and phase shift, by contrast, are relative parameters and require an answer to the question-delayed or shifted from what? In both the time delay and polarity inversion case, the musical series of spikes remains unaltered in shape even if upside down. If we sort the musical signal, on the basis of its pulse width and steepness, to feed a multi-way speaker, it is possible to end up with a different polarity at each driver or a different time delay to each driver, with audible consequences which make evaluating such speakers an art rather than a science and which explain why polarization and controlling delay are important to listeners and designers. If, instead of music, one considers a continuous pure single tone i.e. a sinewave, then it is convenient to speak of a relative phase angle between two such identical waves rather than a time delay. Since sinewaves are smooth, it is pretty hard to find a distinguishing mark from which to measure a delay and one can only see polarity inversion if one has simultaneous access to both waves so as to compare them. When the time delay between two sinewaves is half the period of the tone, or, in other words, when the phase angle between them is 180 degrees, one sinewave looks like the polarity inversion of the other (but looks are deceiving since polarity inversion posits no time delay) and thus the misnomer 'phase inversion' was born. Any pulse waveform, including music, can be thought of as a summation of innumerable waxing and waning sinewaves of differing frequencies, starting/ending times and phases, and this is a useful tool for mathematicians and physicists. But when someone in high-end audio uses the word 'phase', they almost invariably mean either polarity or time delay. The term phase or phase angle is singular and is normally applied only to single frequency continuing waveforms. However, for diehards, it is true that if you phase shift every sinewave component of a complex musical signal by 180 degrees, you will invert the polarity of (as well as slightly advance or delay) the entire signal. But then one should use the plural as in inverting the phases of a loudspeaker input signal. |
